1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) valve to be driven by an actuator and placed in an exhaust gas recirculation passage of an exhaust gas recirculation (EGR) apparatus.
2. Related Art
As the above type of technique, for example, JP-A-2008-202516 discloses an exhaust gas recirculation vale (EGR valve) 31 of a poppet valve structure as shown in FIG. 24. This EGR valve 31 includes a housing 33 formed with a passage 32 for EGR gas, a valve seat 34 provided in some place in the passage 32, a valve element 35 provided to be seatable on the valve seat 34 to form an EGR gas measuring section between the valve element 35 and the valve seat 34, a valve stem 36 provided having one end at which the valve element 35 is provided, and an actuator 37 to perform a stroke movement of the valve element 35 together with the valve stem 36 in an axial direction. Herein, the valve seat 34 is of a circular annular form and has a valve hole 34a at the center. The valve element 35 has a nearly umbrella-like shape. By stroke movement of the valve element 35 together with the valve stem 36 caused by the actuator 37, an opening area of the measuring section, that is, an opening degree of the valve element 35 is changed to adjust a flow rate of EGR gas in the passage 32. Accordingly, the gas flow-rate characteristics of the EGR valve 31 is determined mainly by the shapes of the valve seat 34 and the valve element 35. FIG. 24 is a longitudinal sectional view of the EGR valve 31 in a fully closed state.
Meanwhile, the present applicants analyzed a flow of EGR gas in the above type EGR valve and confirmed that the shapes of the valve seat and the valve element designed in an appropriate relationship could adjust a flow rate of EGR gas at high resolution (with accuracy) in a low opening region and increase a maximum flow rate of EGR gas in a high opening region. FIG. 25 is a sectional view showing the relationship of the shapes of the valve seat 34 and the valve element 35. Herein, in FIG. 25, the valve seat 34 has an annular ring shape formed with a valve hole 34a at the center and the valve element 35 has a nearly umbrella-like shape. In FIG. 25, the valve seat 34 has a first end face 34b and a second end face 34c, each of which is planar, as upper and lower surfaces in an axial direction of the valve seat 34. The valve hole 34a is defined by an inner circumferential surface 34d about half of which is tapered toward a downstream side of EGR gas flow, that is, the inner circumferential surface 34d has a tapered shape with an inner diameter increasing toward the first end face 34b. In FIG. 25, furthermore, the valve element 35 includes a minimum-diameter portion 35a and a maximum-diameter portion 35b so that the maximum-diameter portion 35b is located more upstream in the passage 32 than the minimum-diameter portion 35a. An outer peripheral surface 35c of the valve element 35 is curved to be recessed near the minimum-diameter portion 35a and further curved to protrude near the maximum-diameter portion 35b. In the fully closed state, the valve element 35 is placed such that the minimum-diameter portion 35a is located inside in the valve hole 34a and a portion near the maximum-diameter portion 35b is in contact with an inner circumferential edge of the second end face 34c of the valve seat 34, thereby closing the valve hole 34a. 